In the production of tubes from mixtures of rubber and/or similar plastic material for use as bicycle or motorcycle tubes a long tube blank is continuously extruded. This extrusion is carried out with extrusion heads which produce either one, or two parallel tube blanks. For the production of bicycle or motorcycle tubes the long tube blank is cut into lengths which are bent in circular form and the ends bonded together. Modern production and vulcanizing methods provide for uniting the cut ends on impact. By this uniting of the ends of the cut blank there is produced a tube which, as compared with earlier overlapped joints, uses less material and produces a homogeneous tube which is quiet-running in use.
A prerequesite for this production is that the production precision of the cut extruded tube blank is extremely high so that the butt joint is tight but moreover is possible. For this joining of the ends, the diameter and wall thickness of both ends of the cut tube blank must be very exact. Heretofore this high precision has been attained by hand control of the production mechanism for each individual cut tube blank on the ground of great experience of the service person. However, it was not achieved that in the production of thousands of tube blanks the wall thickness and the cross section dimensions for all of the blanks remained the same. For example, after the production of some hundreds of cut tube blanks substantial dimension differences could be detected between the first, the hundreth and the five hundreth tube blank. That is meaningless for the use of the tubes in power vehicle tires but not for a commercial production.
Tubes for these purposes are on many grounds, principally material cost and short vulcanizing time, made very thin walled. This makes keeping the wall thickness and the cross section dimensions over long extrusion times still more difficult. By reason of this it is indespensible, in particular for thin wall tube blanks, to clean the mixture before leaving the extruder i.e. by pressing it through sieves or strainers so that no production rejects arise through almost unavoidable mixture impurities. A single grain of sand in a thin wall tube can have devastating results. On these grounds, the earlier usual mixture preheating on roller mills has, in recent times, been abandoned in order to avoid the mixture contamination from intermittent operation of the extruder and thereby increase the uniformity of the product.
The vulcanizable or polymerizable mixture of rubber and/or plastic coming from the extruder for processing is very weak, highly viscous and also temperature sensative. In order to prevent the extruded tube blank, which immediately after extrusion becomes oval and has a tendency to lie flat, from sticking together, the tube blank is supported internally with air and thereby also powdered. For this purpose an air-powder mixture is continually introduced through the mandrel of the extrusion head. Excess air must be removed from the interior of the tube blank through a parallel path in order to prevent a blowout of the extrusion. There is thus a problem of maintaining the supporting air in the tube constant as the over pressure with respect to ambient air can be only a few hecto-pascal and must be held constant with great accuracy. For holding the pressure constant, an accurate pressure valve and in particular a sensitive water seal is used.
Holding the supporting air pressure constant is in itself not sufficient as the production parameters change during the production as the sieve is gradually clogged with the collected dirt particles and hence less material is passed through the sieve and the material pressure in front of the sieve increases, with a resulting change in the temperature of the material which alters the viscosity of the mass inter alia and this leads, with constant inner pressure of the tube, to an alteration in the tube cross section.